Genetic analysis in zebrafish is a powerful approach for identifying genes that direct vertebrate development (1-3). Since the completion of the large-scale chemical mutagenesis screens in 1997, the phenotypic and molecular characterizations of many mutations have been reported (4-16).
Analyses of mutations that affect early developmental processes, such as the specification of the embryonic axes and germ layers, have been particularly rewarding (7, 10, 17-27). Recently, related work with mutations that affect organogenesis has led to the recognition that the zebrafish is an important model system for biomedical research (28-31). Given the many aspects of organ physiology that have been conserved during vertebrate evolution, genetic screening to assay organ function in the optically transparent zebrafish is a valuable approach to understanding a variety of metabolic processes and disorders in vertebrates.
By zebrafish chemical mutagenesis screening, nine recessive lethal mutations that perturb development of the digestive organs were identified (2, 31). Although the mutants were identified using morphological criteria, their phenotypic analysis suggests that in some cases the affected genes regulate developmental processes that are relevant to digestive physiology and other aspects of vertebrate metabolism.
Through the analysis of these and other zebrafish mutants, the limitations inherent to genetic screens that are based solely on morphological criteria became apparent. First, not all organs are readily distinguished in zebrafish larvae, and mutations that perturb organ morphology are often overlooked. Second, since it is difficult to visualize specific cell populations within many larval organs, mutations that affect the development or function of these cells can be overlooked as well. Third, despite the transparency of the zebrafish larva, the function of few organs can be effectively assayed by visual inspection alone.
For these reasons, it was concluded that, in most instances, morphology-based screens are best suited for the identification of genes that regulate specification and patterning of embryonic structures. By contrast, screens designed to address biomedical concerns are most effective when they assay physiological processes directly.
Within the past few years, the discovery and analysis of zebrafish mutants affecting organogenesis has confirmed an important role for the zebrafish in biomedical research. The ability to apply high throughput genetic analyses to vertebrate organ physiology using this model system is unprecedented and will undoubtedly, over time, lead to the discovery of many genes that regulate vertebrate organ development and physiology. Such zebrafish research will complement research in other vertebrate model systems.
By conducting a mutagenesis screen using fluorescent lipids, an undertaking not feasible with standard zebrafish screening strategies, the power of high throughput genetic analysis can be applied to lipid metabolism. This has important implications for human diseases such as, but not limited to, cancer, inflammatory and cardiovascular diseases, and congenital and acquired diseases of the intestine and liver.
The fluorescent phospholipase A2 (PLA2) substrates described in the present invention are the first prototypes in this class of reagents. Although lipid metabolism in the digestive tract is complex and involves multiple organs the present invention discloses a method of assaying this pathway since gall bladder fluorescence represents one of the last steps in lipid processing. Because they serve as reporters of lipid processing, the fluorescently-tagged reagents of the instant invention provide a sensitive assay for a wide range of digestive developmental and physiological processes including, but not limited to, swallowing; lipid digestion, absorption, and transport; esophageal sphincter function; intestinal motility; organogenesis of the mouth and pharynx, esophagus, intestine, liver, gallbladder and biliary system, and exocrine pancreas and ducts; and the cellular and molecular biology of PLA2 regulation, polarized transport, and secretion.
Given the shared features of lipid processing in mammals and teleosts (82, 105), zebrafish mutagenesis screens using lipid reporters can be used to identify genes with functions relevant to human lipid metabolism and disease. Moreover, since both mammals and teleosts metabolize lipids in an analogous manner, the high throughput screens and fluorescent lipids disclosed in the instant invention can be employed using a variety of vertebrate model systems, including but not limited to, rodents, amphibia, and fish. The present invention involves utilizing fluorescent lipids to screen for phenotypes representing perturbations of lipid processing; to screen for mutations of specific genes that lead to disorders of phospholipid and/or cholesterol metabolism; and to screen for compounds designed to treat disorders of phospholipid and/or cholesterol metabolism, such as, but not limited to cancer, inflammatory and cardiovascular disease, and congenital and acquired diseases of the intestine and liver.